1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a method of operating a control rod drive mechanism for performing single-step multiple repositionings of a control rod cluster assembly during a single fuel cycle of the nuclear reactor.
2. Description of the Prior Art
In a commercial nuclear reactor, heat, from which steam and ultimately electricity are generated, is produced by fissioning of a fissile material such as enriched uranium. This fissile material, or nuclear fuel, is typically contained within a nuclear core made up of a multiplicity of fuel rods supported in a plurality of nuclear fuel assemblies, coextensively arranged in a spaced parallel array.
Movable control rods are dispersed throughout the core to control the fission process. The control rods generally comprise a plurality of elongated rods containing neutron absorbing materials which fit in longitudinal openings defined in the fuel assemblies and among the fuel rods by guide thimbles of the fuel assemblies. The guide thimbles thus guide the control rods during their movement into and out of the core. Inserting a control rod into the core adds more absorber material and, hence, decreases the nuclear reaction; conversely, withdrawing a control rod removes absorber material and, hence, increases a nuclear reaction and thereby the power output of the core. The nuclear reactor core and the control rods are positioned within and supported by a reactor vessel through which a reactor coolant flows.
The control rods are supported in cluster assemblies moved into and from the nuclear core by control rod drive mechanisms which, in turn, are mounted by an upper internals arrangement located within the nuclear reactor vessel above the nuclear core. Typically, a reactor pressure vessel is pressurized to a relatively high internal pressure. The control rod drive mechanisms operate within the same pressure environment that exists within the reactor pressure vessel. Hence, the control rod drive mechanisms are housed within pressure housings of the upper internals arrangement which are tubular extensions of the reactor pressure vessel.
One of the more commonly used types of control rod drive mechanisms is referred to as a "magnetic jack" With this type of mechanism, the control rods are jacked into and from the nuclear core in a series of motions each involving moving the control rod a discrete incremental distance or "step"; hence, such movement is commonly referred to as stepping of the control rods. This magnetic jack type of mechanism is illustrated and described in U.S. Patents to Frisch (3,158,766) and DeWesse (3,992,255) which are assigned to the assignee of the present invention.
This magnetic jack type of control rod drive mechanism includes three electromagnetic coils and armatures or plungers which are operated to raise and lower a drive rod shaft and thereby the control rod cluster assembly The three coils are mounted about and outside of the pressure housing. Two of the coils actuate respective plungers of movable and stationary grippers contained within the housing The third coil actuates a lift plunger connected to the movable gripper. Actuation of the movable and stationary plungers, in turn, operate sets of circumferentially spaced latches which grip the drive rod shaft having multiple axially-spaced circumferential grooves. The stationary gripper latches are actuated to hold the drive rod shaft in a desired axial position. The movable gripper latches are actuated to raise and lower the drive rod shaft. Each jacking or stepping movement of the control rod drive mechanism moves the drive rod shaft 5/8 inch. (1.58 cm). The jacking or stepping movement is thus accomplished by the operation of the three sets of axially spaced electromagnetic coils to actuate the corresponding stationary, movable and lift plungers so as to alternately and sequentially grip, move and release the control rod drive shaft of the respective mechanism.
The construction and stepping mode of operation of the magnetic jack type of control rod drive mechanism as used heretofore have certain drawbacks. The drawback associated with the construction of the control rod drive mechanism derives from the pivotal mounting geometry of the latches of the stationary and movable grippers of the mechanism. These latches each have either one or a pair of teeth which engage within either a single groove or a pair of adjacent grooves in the drive rod shaft. Because the latches pivotally move in arcuate paths toward and away from drive rod shaft, the teeth are placed at locations on the latch body farthest from the pivotal axis of the latch. This latch teeth placement results in the generation of a moment load through the body of the latch which over time tends to cause cracking at the root of the teeth and eventual failure of the latch. Further, due to the limited number of teeth, the maximum being two, the effective wear life of the latch is limited.
The drawback associated with the mode of operation of the control rod drive mechanism derives from the particular scheme employed in repositioning of the rod drive shaft and thereby each control rod cluster assembly during each nuclear reactor fuel cycle. During normal reactor operation, the control rod drive mechanisms hold the cluster assemblies of control rods withdrawn above the reactor core within guide tubes extending upwardly from and in alignment with the guide thimbles of the fuel assemblies. The force produced by upward flow of coolant causes a surface area of the each control rod wall to contact and rub against the inside of its associated guide tube. Eventually, the wall thickness of the control rod at the area of contact will decreased below allowable limits requiring replacement of the control rod.
To avoid occurrence of rubbing contact wear at a single surface area of the control rod walls and thereby extend the useful life of the control rods, the drive rod shafts of the control rod drive mechanisms and thereby the control rods of the cluster assemblies are moved three steps at every refueling which occurs at the end of each fuel cycle of the nuclear reactor. Ideally, a one-step relocation is all that is needed to present a fresh surface area on the control rod to the portion of the guide tube where the wear is taking place. However, the inherent nature of the magnetic jack type of mechanism is that when it is actuated an occasional misstep will randomly occur in which the control rod is not moved. This presents an unacceptable degree of uncertainty for any given repositioning of the control rods as to whether or not the mechanism actually accomplished relocation of the control rods through one step if the cluster assemblies are only intended to be moved one step. If one-step relocation was not accomplished, or in other words a random misstep occurred, the additional exposure time of the original surface areas of the control rods for another complete fuel cycle at the same elevations would result in unacceptable wear at such areas. The unacceptable risk of a random misstep occurring is offset by the conventional practice of overcompensating and repositioning the control rods through a distance equivalent to three steps, instead of one step. Therefore, even if a one-step misstep occurs, the control rods will still be repositioned through two steps. The current strategy of repositioning the control rods in three-step increments handles the uncertainties of positioning in a simple way; by relocating the control rods an amount that is greater than any potential misstepping uncertainty.
The drawback of the three-step repositioning scheme is that the wear is still poorly distributed throughout the total available clad thickness of the control rods. As an example, assume that a particular nuclear plant has a control rod wear rate that uses 60% of the permissible wear thickness in one fuel cycle. With the three-step repositioning scheme, the control rods would be relocated every fuel cycle so that the permissible minimum level of wear thickness is not exceeded. This effectively "wastes" 40% of the wear thickness at that elevation of the control rods.
Consequently, a need exists for improvements in the construction and mode of operating the magnetic jack type of control rod drive mechanism employed in nuclear reactors so as to overcome the above-described drawbacks.